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Catalytic Asymmetric PictetЦSpengler Reactions via Sulfenyliminium Ions.

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DOI: 10.1002/ange.200701808
Catalytic Asymmetric Pictet–Spengler Reactions via Sulfenyliminium
Martin J. Wanner, Richard N. S. van der Haas, Kimberly R. de Cuba, Jan H. van Maarseveen,
and Henk Hiemstra*
The Pictet–Spengler condensation of tryptamine (or a substituted analogue) with an aldehyde, usually in the presence of
an excess of a Brønsted acid,[1] is the reaction of choice for the
preparation of tetrahydro-b-carbolines (for example, 1), a
structural moiety present in many alkaloids and related
biologically active molecules.[2]
Several strategies have been applied to the synthesis of
tetrahydro-b-carbolines in enantiomerically pure form. It is
possible to start with an enantiomerically pure chiral tryptamine, such as a tryptophan derivative, or aldehyde; alternatively, the use of a chiral auxiliary attached to the nitrogen
atom has been successful. Two remarkable examples are the
cyclization of the N-acyliminium ion 2, in which an a-N,N-
phthaloylamino acid is used as a chiral auxiliary,[3] and the
cyclization of the N-sulfinyliminium ion 3, with the use of
menthyl p-toluenesulfinate as a chiral reagent.[4] The former
procedure proceeds with very high diastereoselectivity, but
the chiral auxiliary can be removed only under relatively
[*] M. J. Wanner, Dr. R. N. S. van der Haas, K. R. de Cuba,
Dr. J. H. van Maarseveen, Prof. Dr. H. Hiemstra
Van’t Hoff Institute for Molecular Sciences
University of Amsterdam
Nieuwe Achtergracht 129, 1018 WS Amsterdam (The Netherlands)
Fax: (+ 31) 20-525-5670
harsh reductive conditions. Lower selectivity is observed with
the second method, but the diastereomers are readily
separable, and the sulfinyl auxiliary can be cleaved easily by
mild hydrolysis.
The first example of a reagent-controlled enantioselective
Pictet–Spengler-type cyclization was reported by Nakagawa
and co-workers.[5] They described the ring closure of nitrone 4
with up to 90 % ee under the influence of a chiral borane
(ca. 2 equiv) as a Lewis acid. Two examples of catalytic
asymmetric Pictet–Spengler reactions have been reported
recently: Taylor and Jacobsen described the cyclization of the
N-acyliminium ion 5 with up to 95 % ee under the catalysis of
an enantiomerically pure thiourea,[6a] and List and co-workers
disclosed the cyclization of the iminium diester 6 with up to
96 % ee under the catalysis of an enantiomerically pure
phosphoric acid derived from binaphthol.[6b] Although both
methods are impressive examples of the power of asymmetric
organocatalysis, the former has the disadvantage that the Nacetyl group is difficult to remove, and the latter is clearly
limited in scope by the requirement of two ester functionalities.
Within our research program on the development of
asymmetric reactions of iminium ions catalyzed by chiral
Brønsted acids, we considered the use of N-sulfenyliminium
ions as intermediates in the Pictet–Spengler condensation.[7]
The sulfenyl substituent was expected to stabilize the
intermediate iminium ion and thus favor Pictet–Spengler
cyclization over undesired enamine formation.[8] Another
advantage of the sulfenyl group is that its ready removal after
the cyclization is ensured. Herein, we demonstrate the
powerful combination of chiral phosphoric acids and Nsulfenyl tryptamines as a useful method for catalytic asymmetric Pictet–Spengler reactions.
The use of phosphoric acids derived from binaphthol as
chiral catalysts in asymmetric synthesis was shown recently by
two Japanese research groups to be highly successful for the
addition of carbon-centered nucleophiles to prochiral iminium intermediates.[9, 10] We set out to use such chiral acids as
catalysts for the generation in situ and cyclization of Nsulfenyliminium ions derived from tryptamine.
The Pictet–Spengler precursors 7 and 8 were prepared
readily by the treatment of tryptamine with the appropriate
commercially available sulfenyl chlorides in the presence of a
base. When the N-(o-nitrophenyl)sulfenyltryptamine 7 was
[**] This research was supported financially by the National Research
School Combination Catalysis (NRSC-C). We thank R. Z. Boerleider
and P. Hauwert for their help in the synthesis of the catalysts.
Supporting information for this article is available on the WWW
under or from the author.
Angew. Chem. 2007, 119, 7629 –7631
2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
stirred with n-hexanal (1.5 equiv) in chloroform in the
presence of the (R)-binaphthylphosphoric acid 9 a (2 mol %)
at room temperature, the tetrahydro-b-carboline 10 was
obtained (Table 1). The reaction was attempted with a
range of chiral phosphoric acid catalysts, 9 b–9 g. In all cases
excellent yields were observed (92–98 %), which underscores
the power of N-sulfenyliminiums to facilitate Pictet–Spengler
reactions. However, the enantioselectivities for the cyclization step were still unsatisfactory: In the best case, the use of
catalyst 9 d led to 10 with a modest 31 % ee.
Table 1: Screening of catalysts for the asymmetric Pictet–Spengler
Substrate 7[a]
t [h]
10: ee [%]
Substrate 8[b]
t [h]
11: ee [%]
[a] Reaction conditions: 7 (0.10 mmol), n-hexanal (1.5 equiv), 9
(2 mol %), chloroform (1.0 mL), room temperature. [b] Reaction conditions: 8 (0.10 mmol), n-hexanal (3.0 equiv), 9 (5 mol %), BHT (5 mg),
3-G MS (150 mg), toluene (1.0 mL), 0 8C. [c] The reaction was continued
until > 95 % conversion. MS = molecular sieves.
We then turned our attention to the bulkier N-tritylsulfenyl-substituted tryptamine substrate 8. The Pictet–Spengler
reaction of 8 with n-hexanal required careful optimization.
The N-tritylsulfenyltetrahydro-b-carboline 11 appeared to be
rather unstable, possibly as a result of homolytic cleavage of
the trityl–sulfur bond. The addition of 3,5-di(tert-butyl)-4hydroxytoluene (BHT) to the reaction mixture as a radical
scavenger prevented the problems associated with the
decomposition of the product. Further improvements
included a solvent change from chloroform to toluene and a
lowering of the reaction temperature to 0 8C. The use of even
lower temperatures had only a marginal effect on the
enantioselectivity of the reaction. Importantly, the addition
of powdered 3-= molecular sieves to remove the water
released during the formation of the iminium salt led to an
improvement in both the yield and the enantioselectivity. The
chiral Brønsted acids 9 a–g were screened under these
optimized conditions. The best result was again obtained
with catalyst 9 d, in the presence of which 11 was formed with
a satisfactory 88 % ee (Table 1).
To avoid the isolation of the labile N-tritylsulfenyl product
11, a one-pot procedure for the synthesis of the tetrahydro-bcarboline 1 a was developed. Thus, the molecular sieves were
removed by filtration after the completion of the reaction.
First thiophenol (1.2 equiv) and then HCl (4 equiv) in
dioxane (4 m) were added to the filtrate, and the resulting
mixture was stirred overnight to give the hydrochloride salt of
the pure tetrahydro-b-carboline as a precipitate. This salt was
collected by filtration and then neutralized to provide pure 1 a
in 87 % yield and with 84 % ee (Table 2). Comparison of the
sign of the specific rotation of the product with literature
Table 2: Scope of the reaction with respect to the aldehyde substrate.[a]
t [h]
Yield [%][b]
ee [%][c]
1 g[d]
1 h[d]
[a] Reaction conditions: 8 (0.10 mmol), aldehyde (3.0 equiv), 9 d
(5 mol %), BHT (5 mg), powdered 3-G MS (150 mg), toluene (1 mL).
[b] Yield of the isolated product after two steps. [c] Determined by
H NMR spectroscopy with (R)-1-(9-anthryl)-2,2,2-trifluoroethanol.[11] [d]
9 d: 10 mol %.
data[3] indicated that (S)-1 a was formed as the major
enantiomer when (R)-9 d was used as the catalyst.
Next, the scope of the reaction with respect to the
aldehyde substrate was investigated. Fast reactions with fair
enantioselectivities occurred with a-unbranched aldehydes,
except in the case of acetaldehyde (Table 2). The best result
was obtained with phenylacetaldehyde, the reaction of which
led to the b-carboline 1 f in 90 % yield and with 87 % ee. The
2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2007, 119, 7629 –7631
aromatic aldehydes tested reacted more slowly; nevertheless,
the corresponding products 1 g and 1 h were furnished in
satisfactory yields and with satisfactory ee values.
As an indication of the scalability of the procedure, 1 a was
formed with the same ee value of 84 % and in comparable
yield when the reaction was carried out on a 3-g scale. When
the catalyst loading was lowered to 1.0 mol %, the Pictet–
Spengler reaction still gave 1 a with 84 % ee, but took 18 h to
reach completion.
In conclusion, an efficient catalytic asymmetric synthesis
of tetrahydro-b-carbolines[12] has been developed on the basis
of the Pictet–Spengler condensation of N-sulfenyltryptamines
with a wide range of aldehydes. The ease of both the
introduction of the sulfenyl substituent and its acid-mediated
removal make this method attractive. Present studies are
directed towards the improvement of the enantioselectivity of
the reaction by further fine-tuning of the sulfenyl substituent
and chiral phosphoric acid catalyst.
Received: April 24, 2007
Published online: August 14, 2007
Keywords: asymmetric catalysis · chiral phosphoric acids ·
enantioselectivity · Pictet–Spengler reaction · sulfenamides
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The extension of this method with N-sulfenyliminium ions to
isoquinoline synthesis is presently under investigation.
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asymmetric, reaction, catalytic, pictetцspengler, ions, via, sulfenyliminium
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